I. Field of the Invention
This invention relates to a process for the production of dielectric mixtures, useful for preventing or diminishing the formation of carbon in dielectric fluids during electrical discharges therein.
II. Description of the Prior Art
During the operation of electrical equipment, such as switches, circuit breakers, transformers, and the like, arcing, sparking or glow discharges usually or occasionally occur, especially at higher voltages. Dielectric materials are commonly employed to reduce or prevent the possibility of such arcing, sparking and glow discharges. For example, solid insulators, such as ceramics or resins, may be used to support or surround electrical conductors. Or, fluid dielectric materials, such as oils or gases, may be used to insulate electrical conductors.
A related problem involves the breakdown of carbon-containing dielectric materials. During arcing, these materials tend to decompose and form carbon, a non-volatile solid, which, being an electrical conductor, not only shortens the gap between conductors, but also eventually leads to carbon bridge short circuits, or deposited carbon tracks. This is a serious problem which has plagued the electrical industry for years.
As used herein, arc interruption includes arc suppression and arc quenching, and refers to preventing or reducing arcing between electrodes. Carbon formation suppression refers to preventing the formation of carbon during arcing. Suppression of carbon formation also prevents formation of conducting carbon tracks or deposits of non-volatile carbon on insulating surfaces. Such deposits are known to produce regions of non-uniform electric fields which result in a decrease in the dielectric strength of the system.
Sulfur hexafluoride (SF.sub.6) is well-known as an excellent gaseous dielectric. See, e.g., U.S. Pat. No. 3,059,044, issued to R. E. Friedrich et al., Oct. 16, 1962. It is unique in its electric arc interrupting properties. However, SF.sub.6 does have a few inherent limitations: low vapor pressure at low temperatures, comparatively high freezing point (-50.6.degree. C) and relatively high cost.
For some years, it has been known that certain electronegatively substituted carbon compounds (halogenated alkanes) are also highly useful fluid insulators in electrical apparatus. Typical examples are dichlorodifluoromethane (CCl.sub.2 F.sub.2), octafluorocyclobutane (c-C.sub.4 F.sub.8), hexafluoroethane (C.sub.2 F.sub.6), octafluoropropane (C.sub.3 F.sub.8), decafluorobutane (C.sub.4 F.sub.10), trichlorofluoromethane (CCl.sub.3 F), sym-dichlorotetrafluoroethane (CClF.sub.2 CClF.sub.2), chloropentafluoroethane (CClF.sub.2 CF.sub.3) and chlorotrifluoromethane (CClF.sub.3). While all of the above have reasonably good dielectric strengths, it is difficult to prevent spark-over or other electrical discharge from occurring in apparatus containing these materials when high voltage surges develop. The spark-over or other discharge typically leads to carbon formation.
A patent issued to J. A. Manion, et al., U.S. Pat. No. 3,650,955, issued Dec. 9, 1966, teaches the use of CCl.sub.2 F.sub.2 combined with c-C.sub.4 F.sub.8 as an arc interrupter gas. However, this combination has been observed to evidence extensive carbon formation properties.
Mixtures of SF.sub.6 and CO.sub.2 have been suggested as a potential gaseous dielectric medium. See, e.g., U.S. Pat. No. 3,059,044, issued to R. E. Friedrich et al., Oct. 16, 1962. However, the patent fails to disclose specific proportions of the components.
Mixtures of insulating gases have been previously disclosed; see, e.g., U.S. Pat. No. 2,173,717, issued Sept. 19, 1939, which discloses mixtures of a gas such as nitrogen or carbon dioxide with other materials such as CCl.sub.2 F.sub.2, and U.S. Pat. No. 3,281,521, issued Oct. 25, 1966, which discloses mixtures of nitrogen, CCl.sub.2 F.sub.2 and SF.sub.6. However, there is no disclosure or suggestion in these patents as to whether carbon formation, which is well-known to occur when carbon-containing gases are exposed to arcing or corona conditions, can be suppressed.
Perhalogenated fluids, including SF.sub.6 and perhalogenated alkanes, have been absorbed on molecular sieves (zeolites), which are then incorporated as fillers in organic insulators; see U.S. Pat. No. 3,305,656, issued to J. C. Devins, Feb. 21, 1967. During high voltage operation, voids in the insulation are filled by the perhalogenated fluid, which then serves as an arc interrupter.
Attempts have been made to develop gaseous dielectric compositions as carbon formation suppressants. For example, B. J. Eiseman, U.S. Pat. No. 3,184,533, issued May 18, 1965, teaches the use of an oxygen-containing oxidizing agent, such as SO.sub.2, N.sub.2 O and NO, to suppress carbon tracing of certain electro-negatively substituted carbon compounds, such as saturated polyhalohydrocarbon compounds, saturated perhalohydrocarbon compounds, saturated perfluoroethers and the like. However, none of these oxidizing agents is desirable because of their corrosive nature, toxicity, and/or chemical reactivity.
In general, any attempts to suppress carbon formation in carbon-containing dielectric gases exposed to arcing or corona conditions by use of a diluent gas requires a high percentage of the diluent gas. Since the well-known diluent gases of nitrogen, carbon tetrafluoride and the like usually have a low dielectric strength, then any gaseous dielectric mixtures employing these diluent gases will consequently have a dielectric strength intermediate the dielectric gas and the diluent gas.
There remains in the art a need for an efficient gaseous dielectric composition that evidences superior carbon suppression properties.